<!DOCTYPE html>
<html lang="zc-CN">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
    <script>
        let year = +prompt('请输入年份')
        let month = +prompt('请输入月份')
        if(month===1 || month===3 || month===5 || month===7 || month===8 || month===10 || month===12) {
            document.write('这个月有31天');
        }else if (month===4 || month===6 || month===9 || month===11) {
            document.write('这个月有30天');
        }else if (month===2 && year % 4===0 && year % 100 !==0 || year % 400 ===0) {
            document.write('今年2月是29天');
        }else {
            if(month===2)
            document.write('今年2月是28天');
            else {
                document.write('请输入正确的月数');
            }
        } 

    </script>
</body>
</html>